The invention relates to a telephone set that enables the telephone line connected thereto to be seized in calling or answering mode, and released, both by manually lifting the handset off the hookswitch thereof as well as by means of a command produced for example by a fleeting touch of an automatically returned button or by the action of a suitable device, without going off-hook.
More particularly, the invention concerns telephone sets which include at least one circuit requiring an initial determined step to be gone through which requires advance powering prior to being capable of operating, for example a microprocessor-type circuit.
Seizure of a telephone line connected to a telephone set by a user calling from such a set can be conventionally done by the user simply lifting the handset or by the creation of a command as indicated above.
Answering a call terminating at a telephone set, via the line to which the set is connected is achieved in identical fashion by manually going off-hook or by sending a command which can be the above-mentioned seizure command.
In both cases, seizure for an originating or terminating call is expressed in practice by closing a loop made up by the two wires of the line between the telephone set and the remote central office that hosts it; current is then supplied to the set by the line as long as the loop stays closed, allowing the set to be powered, at least partially. In numerous cases, the telephone set is no longer powered when the loop is reopened by manually going on-hook with the handset acting on the hookswitch contacts associated with the set and/or by the action of an on-hook command which can possibly be identical to the seizure command and/or be produced by the same device.
However, some circuits and notably processors fitted into telephone sets for providing a variety of various operating possibilities require imperatively to be powered in order to be set to a determined initial state before being able to be operational.
This is hence a restrictive condition when such a circuit incorporated in a telephone set is only powered by the line to which the set is connected and, consequently, only from the moment where the latter itself is appropriately powered, since, without special arrangements, certain actions performed at set level via, for example, a microprocessor would not be able to be handled as the latter would not be in a position to process them.
The tendency is to however fit such circuits and in particular microprocessors into telephone sets in order to increase operating possibilities to the advantage of users.